1. Field of the Invention
The present invention relates to a watercraft with an improved waterjet propulsion system. The present invention further relates to a waterjet propulsion system with an improved steering nozzle design.
2. Technical Background
It has been determined that driving a hull throughout the water is more efficient in midrange speeds using a waterjet than the traditional screw (propeller). The waterjet, which is basically a water pump, utilizes an axial or centrifugal or mixed pump to rapidly increase the velocity of the water and expel the water through a nozzle. The intake grate acts not only as a water access point, but a focus point, which can assist in propulsion once in motion.
The immediate benefits of a waterjet propulsion system are increased maneuverability, impeller protection and access to shallow draft hull designs. Waterjet powered boats exhibit incredible maneuvers such as 180 degree turns or stops in less than a boat length, sides “walking” and in some cases beach landings without damage to the propulsion system. Since water is taken from an intake grate, the system can be mounted flush with the bottom of the boat almost eliminating power plant damage from debris and bottom strikes.
With a waterjet propulsor drive, water is drawn in through the bottom of the watercraft and ejected in a stream out the back. The reaction to this movement of water is the propulsive force that moves the boat. Near the back of the propulsor is a nozzle, which serves three functions. It accelerates the stream by reducing its diameter, and it can be turned from side to side to deflect the exiting stream to apply a component of side force on the aft part of the boat, or trim of the watercraft. The nozzle is typically connected to a steering wheel or mechanism on the watercraft.
A key element in the performance of the watercraft with a waterjet propulsor is the trimming of its steering nozzle. Nozzle trim involves aiming the steering nozzle either up or down within a range of approximately 20 degrees about the longitudinal axis of the waterjet propulsor. This adjustment of the nozzle's vertical angle has a dramatic impact on watercraft performance over the entire range of watercraft speed. Lowering the steering nozzle (aiming it down) causes the watercraft's stern to rise, which forces the bow further into the water. This increases the wetted surface area, which significantly increases the drag and limits planing of the watercraft. For this reason, this condition is rarely used. Raising the nozzle (aiming it up) causes the watercraft's stern to lower down into the water, which causes the bow to be lifted out of the water. This increases the aerodynamic lift on the watercraft hull, which partially lifts the watercraft hull out of the water. Less wetted surface area contact between the watercraft and the water significantly reduces hydrodynamic drag and increases top speed.
Raising the steering nozzle (aiming it up) is inefficient since it causes the waterjet to arch over the surface of the water directly behind the watercraft. This arch is produced by aiming the waterjet directly into the air or by a deflection of the waterjet encouraged by the wake profile directly behind the watercraft. In either case, the waterjet arch is referred to as a “rooster tail”. One disadvantage of the “rooster tail” is that the waterjet discharge direction is not aimed entirely in the direction of the desired watercraft motion. The angle of the steering nozzle can be as high as 10 degrees off the longitudinal axis of the waterjet propulsor, which can reduce the available thrust. Further for military applications requiring a high degree of stealth, large rooster tail exaggerates the visible radar signature of the combatant watercraft. Large wake and especially rooster tail results in increased visibility and a higher probability for detection in environments monitored by aided and even unaided observers.
For waterjet-propelled watercraft, rooster tail has also been found to reduce the top speed capability of the waterjet propulsor system. This occurs despite the fact that off-axis aiming of the waterjet to increase watercraft planing. Waterjet nozzles have been developed to vary the exit area of the nozzle to obtain greater thrust or top speed. This has typically been done by adding circular ring structures into the nozzle that reduce the exit area. These devices only, however, provide benefits either in low speed acceleration or at top performance speeds—not both resulting in a compromise on either end of the performance spectrum.
It is therefore an object of this invention to provide for waterjet-propelled watercraft with increased efficiency. It is further an object of this invention to provide for a waterjet-propelled watercraft with reduced wake or “rooster tail”. It still is an object of this invention to provide for a waterjet-propelled watercraft with improved performance across the performance spectrum.